CDL Air Brakes TestInspecting Air Brake SystemsHard
A driver notices that the air pressure gauge shows a steady 125 psi, but the low air warning light is illuminated. The driver has already confirmed the gauge is accurate. What is the most likely cause of this discrepancy?
- AA clogged air filter in the compressor intake.
- BA stuck open air dryer purge valve.
- CA short circuit in the low air warning light's wiring.
- DA faulty pressure protection valve.
Show answer & explanationAnswer & explanation
Correct answer: C. A short circuit in the low air warning light's wiring.
If the air pressure gauge is accurate and showing full pressure (125 psi), but the low air warning light is still on, it indicates an electrical issue with the warning system itself, not a problem with air pressure. A short circuit in the wiring for the warning light would cause it to illuminate incorrectly, even if pressure is adequate.
Why the other options are wrong
- A. A clogged air filter would reduce compressor efficiency, making it harder to build pressure, not cause a false low air warning when pressure is full.
- B. A stuck open air dryer purge valve would cause a continuous air leak, preventing pressure from building or maintaining, which contradicts the gauge showing 125 psi.
- D. A faulty pressure protection valve affects air supply to accessories, not typically the low air warning light when main tanks are full.
False Low Air Warning
When the low air pressure warning device activates despite the air pressure being at a safe operating level.
- Usually indicates an electrical fault in the warning system.
- Could be a faulty pressure switch or wiring issue.
- Verify actual pressure with a reliable gauge.
Memory trick: Full pressure, light's a fright? Electrical short, not air's plight!